In the grating external-cavity lasers, there usually needs to tune the wavelength or frequency of generated laser and such tuning is realized by rotating grating to vary the incidence angle and diffraction angle of light on the grating, or by rotating mirror to vary the diffraction angle of light on the grating.
FIGS. 1, 2 and 3 show three types of grating external-cavity semiconductor lasers respectively. Wherein, FIG. 1 shows a conventional external-cavity semiconductor laser in a grazing-incidence configuration (i.e. the incidence angle is larger than the diffraction angle), also known as Littman configuration; FIG. 2 shows a novel external-cavity semiconductor laser in a grazing-diffraction configuration (i.e. the diffraction angle is larger than the incidence angle) proposed by the same applicant in Chinese patent application No. 200810097085.4; and FIG. 3 shows a conventional external-cavity semiconductor laser in Littrow configuration, in which there has no mirror and thus tuning is done only by rotating the grating.
Hereinafter, the basic configuration and principle of grating external-cavity laser will be discussed taking grating-feedback external-cavity semiconductor laser (ECDL) as an example. As shown in FIGS. 1-3, the semiconductor laser diode is represented as LD, the aspheric collimating lens is represented as AL, the grating is represented as G, the feedback mirror is represented as M, the normal of the grating is represented as N, the incidence angle of light on the grating is represented as θi, the diffraction angle of light on the grating is represented as θd, the difference between the incidence angle and the diffraction angle is Δθ, that is, Δθ=θi−θd, and Δx is the optical path increment generated by the optical elements in the cavity (e.g. the gain media of the aspheric collimating lens and the LD).
In the grazing-incidence configuration shown in FIG. 1 and the grazing-diffraction configuration shown in FIG. 2, a laser beam emitted by the laser diode LD is incident on the diffraction grating G after the collimation of the aspheric collimating lens AL. The first order diffracting light from the grating G is normally incident on the feedback mirror M, which, after the reflection of the feedback mirror M, is re-diffracted by the grating along the path collinear with the incident light and in the direction opposite to the incident light, and then returns to the laser diode through the aspheric collimating lens AL following the original optical path.
In the Littrow configuration shown in FIG. 3, a laser beam emitted by the laser diode LD is incident on the diffraction grating G after the collimation of the aspheric collimating lens AL. The first order diffracting light from the grating G directly returns to the semiconductor laser diode through the aspheric collimating lens AL along the path collinear with the incident light and in the opposite direction of the incident light following the original optical path. It can be seen that, in the Littrow configuration, the incidence angle is equal to the diffraction angle of light on the grating, that is, θ=θd=θ, and thus Δθ=0.
In order to illustrate the tuning principle of external-cavity semiconductor lasers, a Cartesian coordinate system xOy is introduced in the figures, wherein the point O represents the intersection point of a laser beam emitted from semiconductor laser diode LD and the diffraction surface of grating G in its original position; the x axis runs through the point O and its direction is collinear with and opposite to that of the light emitted from LD; and the y axis runs through the point O upward and is perpendicular to the x axis.
The three planes of the equivalent LD rear facet, the diffraction surface of the grating G and the reflection surface of the mirror M are all perpendicular to the xOy coordination plane. The intersection line of the plane on which the diffraction surface of the grating lies and the xOy coordination plane is represented as SG, and the point O is on the intersection line; the intersection line of the plane on which the equivalent LD rear facet lies and the xOy coordination plane is represented as SL, which is separated from the point O by a distance l1; and the intersection line of the plane on which the reflection surface of the feedback mirror M lies and the xOy coordination plane is represented as SM, which is separated from the point O by a distance l2.
In the grazing-incidence configuration shown in FIG. 1 and grazing-diffraction configuration shown in FIG. 2, the optical distance between the point O and the equivalent LD rear facet and the optical distance between the point O and the feedback mirror M, i.e. the lengths of the two sub-cavities of the grating-external cavity, are represented as l1 and l2 respectively, and the whole optical length of cavity of the semiconductor laser is represented as the sum of them, i.e. l=l1+l2. In the Littrow configuration shown in FIG. 3, the actual optical cavity length of the laser is l1, that is, the distance between the point O and the equivalent LD rear facet.
When rotating the grating G or the mirror M to perform tuning, the rotational axis is perpendicular to the xOy coordinate plane, and the intersecting point of the rotational axis and the xOy coordinate plane (i.e., a rotation center) is denoted as P(x,y) in FIGS. 1-3. For convenience three distance parameters u, v and w are introduced, wherein u represents the distance between the rotation center P and the intersection line SM; v represents the distance between the rotation center P and the intersection line SG; and w represents the distance between the rotation center P and the intersection line SL. The signs of u, v and w are defined as follows: they are positive when the light and the rotation center are on the same side of the respective plane intersection lines, and they are negative when the light and rotation center are on the opposite sides of the respective plane intersection lines respectively. The distance v or u does not change when the grating G or the mirror M is rotated around the point P.
In grating external cavity semiconductor laser, there are two essential factors for laser wavelength or frequency determination:
1. frequency selection determined by the values of incidence angle and diffraction angle of the light on the grating and their variations;
2. frequency selection determined by the values of the cavity length of the equivalent F-P cavity formed by SL, SM and SG and their variations.
During the rotation of the grating or mirror around the rotation center P, both the frequency selection of the grating and the frequency selection of the F-P cavity change. In general, those changes are not synchronous, which will cause mode-hopping of the laser mode, thus will disrupt the continuous tuning of laser frequency, and hence, resulting in a very small continuous tuning range without mode-hopping, e.g., 1 to 2 GHz.
In order to achieve synchronous tuning of laser frequency or wavelength, i.e., achieve a large range of continuous frequency tuning without mode-hopping, the rotation center P of the grating G or the feedback mirror M needs to be selected purposefully.
Assuming that the grating or the mirror was rotated by an angle α with respect to its original position, the phase shift Ψ of laser beam after one round trip within the F-P cavity is:ψ=ψ0+A(α)·[B·sin α+C·(1−cos α)]  (1)
wherein, Ψ0 is the original one round trip phase shift of the beam before the rotation tuning, A(α) is a function of the tuning rotation angle α. Ψ0, B and C are functions that are irrelative to the angle α. Ψ0, A(α), B and C relate to the original parameters of the external-cavity semiconductor laser, including original angles (for example, original incidence angle θi, original diffraction angle θd etc.), original positions (for example, original cavity lengths l10 and l20, and original distances u0, v0 and w0), and grating constant d, and the like. When full synchronous tuning conditions are satisfied, the phase shift Ψ should be independent of the rotation angle α, and thus, both B and C in Eq. (1) should be zero.
Here, the distance parameters of the rotation center P0 fulfilling rigorous synchronous tuning should meet:
                    {                                                                                                  u                    0                                    +                                      w                    0                                                  =                0                                                                                                          v                  0                                =                0                                                                        (        2        )            
It is evident that the rotation center P0 satisfying synchronous tuning conditions should lie on the intersection line of the plane on which the grating diffraction surface lies and the xOy coordinate plane; meanwhile, the distance u0 from the rotation center P0 to the plane on which the reflection surface of the mirror lies and the distance w0 from P0 to the plane on which the equivalent LD rear facet lies have the same absolute values and the opposite signs.
For grazing-incidence and grazing-diffraction configuration, the coordinate of the rotation center satisfying synchronous tuning conditions is represented as P0(x0,y0), which meet:
                    {                                                                              x                  0                                =                                                      l                    0                                    ⁢                  d                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                                      i                    /                    λ                                                                                                                                            y                  0                                =                                                      l                    0                                    ⁢                  d                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                                      i                    /                    λ                                                                                                          (        3        )            
Wherein, x0, y0 are abscissa and ordinate of the synchronous tuning rotation center P0 respectively, l0 is the equivalent cavity length of the F-P cavity at the original position, d is the grating constant, θi is the incidence angle of the light on the grating, and λ is the laser wavelength.
FIGS. 4 and 5 show the synchronous tuning of grazing-incidence configuration and grazing-diffraction configuration respectively.
FIG. 6 shows the synchronous tuning of the Littrow configuration. Since there has no mirror in the Littrow configuration, which means that u0=w0, the distance parameter constraint conditions defined in Eq. (2) become:
                    {                                                                              w                  0                                =                0                                                                                                          v                  0                                =                0                                                                        (        4        )            
That is, the synchronous tuning center P0 should at the intersecting point of the lines SG and SL.
Since θi=θd=θ and the actual optical cavity length is l1 in the Littrow configuration, when expressed by coordinate of P0(x0, y0), the distance parameter constraint conditions defined in Eq. (3) become:
                    {                                                                              x                  0                                =                                  l                  ⁢                                                                          ⁢                                      1                    0                                                                                                                                            y                  0                                =                                                      l                    ⁢                                                                                  ⁢                                          1                      0                                                                            tan                    ⁢                                                                                  ⁢                    θ                                                                                                          (        5        )            
It can be seen from the above description that, regardless of whether the coordination parameter or the distance parameter is used, the position of the synchronous tuning rotation center P0 needs to be defined by a equation group consisting of two equations, and the above two constraint conditions must be satisfied simultaneously, which means that there needs two adjustment mechanisms with the independent freedoms in the laser design. Despite for the grazing-incidence configuration, the grazing-diffraction configuration or the Littrow configuration, the position of the synchronous tuning rotation center P0 can not leave from the SG plane on which the diffraction surface of the grating lies, which leads to disadvantages and difficulties in configuration design, adjustment and application of laser, while complicates the mechanical system and increases the instable factors.
In practice, a large continuous tuning range without mode-hopping may be affected by many other factors, for example, whether there is a AR(antireflection) coating applied on the LD surface and the quality of coating and the like. However, a continuous frequency tuning range of hundreds or even tens of GHz may be sufficient for many applications.